Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay, after purification and beneficiation, is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
Crude kaolin clay, as mined, contains discoloring impurities such as oxides of titanium and iron. These impurities are principally responsible for the poor whiteness, brightness and color of the crude clay and are often the reasons for rejecting such clay for certain commercial uses.
The use of kaolin as a coating pigment in the paper industry requires high brightness, proper color and gloss, which necessitates the removal of discoloring impurities. In general, wet beneficiation or purification of the kaolin to remove titanium and iron contaminants involves high intensity magnetic separation, froth flotation, selective flocculation and/or leaching. The kaolin is initially dispersed in water, degritted (generally defined in the industry as removal of particles coarser than about 44 microns) and then beneficiated in slurry form.
High intensity magnetic separation involves the use of a magnetic field to remove the impurities with magnetic susceptibility, such as anatase (TiO.sub.2), rutile, hematite, mica and pyrite. However, this method is not very effective for removing submicron particles, which decreases the ability of high intensity magnetic separation to produce high brightness kaolin products.
Froth flotation has proven to be an efficient method of removing discoloring impurities from kaolin clays. In froth flotation, the impurity is rendered selectively hydrophobic by a collector (e.g., fatty acid, tall oil, hydroxamate, etc.) after activation in some cases (i.e., conditioning with monovalent, divalent or trivalent cations for tall oil flotation). The hydrophobic particles attach to the air bubbles and are separated from the hydrophilic kaolin in a froth flotation cell or column. The flotation process for removal of discoloring impurities from kaolin is described in U.S. Pat. Nos. 3,450,257; 3,979,282; 4,472,271; 4,492,628; 4,629,556; and 5,522,986.
Another variation of froth flotation involves the use of carrier particles to improve fine particle flotation, as described in U.S. Pat. No. 2,990,958. However, due to the very fine particle size of kaolin clays, especially the fine-grained Tertiary kaolins, froth flotation is complicated and can be inefficient and costly.
Selective flocculation has been more successful in beneficiating fine-grained kaolin clays. Selective flocculation involves activation of the discoloring impurity with polyvalent cations (as described in U.S. Pat. Nos.3,371,988; 3,701,417; 3,837,482; and 3,862,027), conditioning with an ammonium salt (as described in U.S. Pat. No. 4,604,369) or fatty acid and polyvalent cations (as described in U.S. Pat. No. 5,535,890) and then flocculating the impurities with anionic polymers. A disadvantage of this process, however, is the relatively low recoveries.
Another process involves leaching of kaolin clay with iron reducing reagents such as zinc or sodium hydrosulfite. This leaching method is limited to removing iron contaminants only. Other known leaching reagents and/or processes are not generally economical for removing titanium impurities.
The effect of particle size distribution on the optical properties of kaolin clay is described in U.S. Pat. No. 2,158,987. Size classification is commonly used to produce the different grades of kaolin for paper coating and fillers, as well as the other kaolin products for paint, plastics, inks, adhesives and rubber.
The prior art has also used selective fractionation or classification of kaolin by sedimentation or centrifugation into different size ranges to produce a coating pigment of improved opacity, brightness and gloss (U.S. Pat. Nos. 2,992,936 and 3,085,894). Another prior art process uses size classification (i.e., defining or desliming) to remove the fine fraction or colloidal particles from delaminated clays and thereby produce a pigment with a narrower particle size distribution (U.S. Pat. Nos. 5,085,707 and 5,168,083). These processes may include a chemical treatment (such as by addition of an amine and aluminum sulfate) to produce the final product. However, these processes have not used classification to improve the efficiency, reduce the cost, lower reagent demand or broaden the application of the beneficiation process in removing discoloring impurities from kaolin clays.
Well known in the industry is the wet processing of kaolin to degrit crude kaolin after dispersing the crude kaolin into slurry form. Degritting involves removing the coarse particles or grit by screening, gravity sedimentation and/or the use of a cyclone separator. Although degritting removes some of the impurities such as rutile, pyrite, marcasite, quartz, muscovite, kyanite, etc., the main objective of degritting is to remove the coarse particles which are not acceptable in kaolin clay formulations used as pigments or fillers.
Due to the limitations of the prior art beneficiation processes in removing discoloring impurities from kaolin clays, there is a need to develop a process that is more efficient and cost effective. Also, there is a need for a process that will more effectively beneficiate the fine-grained kaolin clays.